1. Field of the Invention
The present invention relates to an optical displacement sensor and an optical displacement measurement device and more particularly relates to an optical displacement sensor and an optical displacement measurement device which detect a target object on the basis of the outputs of a plurality of light reception elements for receiving light reflected by the target object when projected light is directed to the target object.
2. Description of the Related Art
As sensors for detecting the presence or absence of a target object by directing light to the target object, there have been known photoelectric sensors and optical displacement sensors. Such photoelectric sensors are sensors which determine the presence or absence of a target object, on the basis of whether or not a light reception device receives projected light emitted from a light projection. Optical displacement sensors are sensors which calculate the amount of displacement of a target object, on the basis of light reflected by the target object when projected light is directed to the target object, and determine the presence or absence of the target object, on the basis of the calculated amount of displacement.
Such optical displacement sensors are capable of determining the presence or absence of objects without being influenced by the colors and the materials of the objects, in comparison with photoelectric sensors which perform determination simply on the basis of the amount of received light, since the amount of received light detected by the light reception device is varied depending on the colors and the materials of the target objects. Further, a displacement sensor including a light reception device constituted by a plurality of light reception elements arranged in a linear shape for receiving light reflected by a target object determines a one-dimensional position of the received-light spot on the light reception device, on the basis of the amount of light received by the respective light reception elements (for example, shown in Japanese Published Patent No. 2006-38571). For example, the distribution of the amount of light obtained by the light reception elements in the direction of the arrangement of the respective light reception elements is obtained and the peak position on the distribution of the amount of light is determined as the position of the received-light spot. The amount of displacement of a target object is calculated on the basis of the determination of the position of the received-light spot obtained in such a manner.
An optical displacement sensor as described above determines the position of a received-light spot on the basis of the distribution of the amount of light obtained from a plural of light reception elements, which can reduce the error caused by the condition of the surface of the target object, in comparison with sensors which employ PSDs (Position Sensitive Devices) for detecting the position of the barycenter of the amount of received light. For example, even when the target object has a mirror surface and the sensor itself is reflected by the object surface or when the object has a shiny surface and the projected light is irregularly reflected by the object surface thereby distorting the distribution of the amount of light and deviating the position of the barycenter of the amount of received light, the aforementioned optical displacement sensor is capable of correctly detecting the presence or absence of the object.
FIG. 18 is a view illustrating the structure of a conventional optical displacement sensor 100, schematically illustrating a state where projected light emitted from a light projection unit 101 is received by a light reception unit 104. The optical displacement sensor 100 is constituted by the light projection unit 101, and the light reception unit 104 constituted by a light reception lens 102 and a light reception device 103. The light reception unit 103 is an image pickup device including a plurality of light reception elements placed in a linear shape for receiving the reflected light from the work 120, and the respective light reception elements output signals corresponding to the amount of received light. The light emitted from the light projection unit 101 and directed to the work 120 is reflected by the work 120 and is converged at a position on the light reception device 103 which is varied depending on the height of the irradiation point on the work 120, namely the position of the irradiation point in the direction of the optical axis of the projected light. The displacement sensor 100 determines a one-dimensional position of the received-light spot on the aforementioned light reception unit 103 on the basis of the outputs of the respective light reception elements and calculates the amount of displacement of the work 120 on the basis of the result of the determination.
FIG. 19 is a view illustrating amount of light received by the respective light reception elements in the optical displacement sensor 100, designating the position on the light reception unit 103 along the horizontal axis while designating the amount of received light along the vertical axis. The curve 131 is a curve indicating a distribution of the amount of received light obtained when the projected light is reflected by the work bench 110, indicating a mound-shaped distribution having a maximum amount of received light at a positional. The curve 132 is a curve indicating a distribution of the amount of received light obtained when the projected light is reflected by a work 120, indicating a distribution having a position at which the amount of received light is peaked (position a2) at the position which is left of the curve 131. The spread of the mound-shaped distribution indicates the size of the received-light spot, and the peak position indicates the position of the received-light spot. The procedure for detecting the presence or absence of the work 120 is as follows. At first, a peak position a2 obtained when the work 120 exists on the work bench 110 and a peak positional obtained when the work 120 does not exists on the work bench 110 are determined, and a threshold value a3 is defined between these peak positions. Then, in actual operation, a peak position determined on the basis of the distribution of the amount of received light, namely the position of the received-light spot, is compared with the threshold value a3 and, on the basis of the result of the comparison, the determination of the presence or absence of the work 120 is performed.
FIG. 20 is a view illustrating the optical displacement sensor 100 placed in a manufacturing line. The displacement sensor 100 is used for detecting the presence or absence of works 120 being transferred in the direction of the line along the work bench 110. In this case, when a work 120 exists at a position beneath the displacement sensor 100 to which the projected light is directed, the peak position in the distribution of the amount of received light is different from the peak position when no work 120 exists at the position. The displacement sensor 100 detects the presence or absence of a work 120, utilizing the fact that the peak position in the distribution of the amount of received light, namely the amount of displacement of the work 120, is varied depending on whether or not the work 120 exists.
FIG. 21 is a timing chart illustrating operations of the optical displacement sensor 100 for detection of works, illustrating detected values of amounts of displacements and sensor outputs indicative of the presence or absence of work 120. The detected value of the amount of displacement is the distance to the irradiation point in the direction of the optical axis of the projected light and is calculated on the basis of the peak position in the distribution of the amount of received light. The detected value of the amount of displacement (the distance b2) acquired when a work 120 exists is smaller than the detected value (the distance b1) when no work 120 exists. The threshold value b3 is a threshold value for use in determining the presence or absence of a work 120 and corresponds to the threshold value a3 regarding the position of the received-light spot (b2<b3<b1). If the detected value of the amount of displacement is decreased to below the threshold value b3, the sensor output is turned on, and the voltage level thereof is switched from a low level to a high level. Further, if the detected value of the amount of displacement is increased to above the threshold value b3, the sensor output is turned off, and the voltage level thereof is switched from the high level to the low level. The displacement sensor 100 determines the presence or absence of a work 120, by making a comparison between the position of the received-light spot, namely the amount of displacement of the work 120, and the threshold value, as described above.